IRC 47 (1972) outlines provisional guidelines for the application of built-up spray grout, a bituminous two-layer base or binder course designed for road pavements. This method serves as an alternative to traditional water bound macadam, particularly in regions with limited water availability or where traffic disruption must be minimized. The code details the materials, aggregate gradation, binder selection, construction methodology, and quality assurance requirements to produce a durable base layer not intended for use as a wearing surface.
Overview
IRC 47 (1972) outlines provisional guidelines for the application of built-up spray grout, a bituminous two-layer base or binder course designed for road pavements. This method serves as an alternative to traditional water bound macadam, particularly in regions with limited water availability or where traffic disruption must be minimized. The code details the materials, aggregate gradation, binder selection, construction methodology, and quality assurance requirements to produce a durable base layer not intended for use as a wearing surface.
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Contents
Structure
IRC 47 introduces the built-up spray grout as a two-layer, bituminous base or binder course alternative to traditional water bound macadam, unsuitable as a wearing surface. It consists of compacted coarse aggregates bound with bitumen after each layer and covered with key aggregates, with a maximum total thickness of 75 mm. Acceptable binders include straight-run bitumen (IS:73), road tar (IS:215), or approved cutback bitumen, selected per climatic requirements. Aggregates must meet cleanliness, strength, durability, and shape criteria as specified in related tables. Construction requires a dry, clean base and binder application rates per prescribed standards.
This code covers provisional specifications for the use of built-up spray grout in road base and binder courses. Construction is prohibited under shade temperatures below 166C or during adverse weather conditions such as wet, foggy, rainy, or dusty environments. The base must be uniformly graded, compacted, and free of surface irregularities. Surfaces need thorough cleaning before binder application, including removal of dust and loose matter, especially on old blacktop. Stabilized or porous bases require a bituminous primer per IRC:16-1965 prior to spray grout application. Binder application rates vary with base type, ensuring optimal adhesion.
Key materials include bituminous binders and aggregates. Binders must be straight-run bitumen (IS:73) of grades 80/100, 60/70, or 30/40; road tar (IS:215) grades R.T.4 or R.T.5; or approved cutback bitumen. Aggregates should be crushed stone, slag, gravel, or similar, meeting physical and durability parameters such as abrasion resistance, impact value, flakiness index, stripping value, water absorption, soundness, and density as detailed in specified tables. Grading requirements for coarse and key aggregates are defined to ensure proper bonding and durability.
Aggregates must be clean, cubical, strong, and free of deleterious substances. Physical properties for both coarse and key aggregates, including Los Angeles abrasion, impact value, flakiness, stripping resistance, water absorption, soundness, and bulk density, are prescribed with maximum permissible limits. Grading curves for aggregates specify sieve sizes and percentage passing to ensure uniformity and suitability for spray grout layers.
The binder types allowed include straight-run bitumen conforming to IS:73, road tar per IS:215, or approved cutback bitumen. Selection of binder grades depends on prevailing climatic conditions. The binder is heated to the specified temperature and applied uniformly at rates defined for the base type, typically between 6 to 10 kg per 10 m8, to bond compacted aggregate layers. This process is essential for achieving adequate adhesion and durability.
Construction must avoid temperatures below 166C and adverse weather. The base surface should be dry, clean, and uniformly graded. Bituminous binder is to be heated and sprayed evenly just before aggregate placement. Aggregates are spread and compacted in two layers, each bonded with binder and finished with key aggregates. Total thickness should not exceed 75 mm. This method is meant for base or binder courses and is unsuitable as a final wearing surface.
The underlying base must be shaped to a uniform grade and free from depressions or potholes, which should be filled and compacted beforehand. The surface has to be dry and free from dust and debris through sweeping, brushing, and blowing. Old bituminous surfaces require thorough cleaning of sand, dirt, and loose particles. Stabilized soils or porous aggregates need a bituminous primer application and curing prior to grout placement. Binder is then applied uniformly just before aggregates are spread.
Coarse aggregates for the initial layer are spread at approximately 0.5 m3 per 10 m8 and hand-packed to a uniform thickness and profile using templates. Compaction is carried out with a three-wheel power roller or equivalent, beginning from edges inward, with surface variations limited to 12 mm over a 3 m straight edge. Rolling is stopped before voids close completely to prevent binder infiltration issues. After compaction, binder is applied at 12.5 to 15 kg per 10 m8 with mechanical sprayers, ensuring the aggregate surface is dry.
Once the first aggregate layer is compacted, the binder is heated and uniformly applied at a rate between 12.5 and 15.0 kg per 10 m8. The binder types include straight-run bitumen, road tar, or approved cutbacks selected by climate considerations. The aggregate must be surface dry before application to ensure proper adhesion. Binder rates for tack coats vary depending on base type, optimizing bonding before the second aggregate layer is placed.
Following binder application, the second layer of coarse aggregates is spread uniformly at the same rate and compacted similarly to the first layer. Rolling is conducted with overlapping passes, starting from edges inward, ensuring that surface deviations do not exceed specified limits. After compaction, a second binder application at 12.5 to 15.0 kg per 10 m8 is uniformly sprayed. This completes the two-layer assembly, maintaining a total thickness under 75 mm.
The second binder application occurs after compacting the upper coarse aggregate layer, using the same binder types and grades as previously specified. Application rates remain between 12.5 and 15.0 kg per 10 m8. After this, key aggregates are applied and compacted to produce a uniform and sealed surface. This final step ensures the structural integrity and bonding of the built-up spray grout base.
Key aggregates are spread over the two compacted aggregate layers bonded with bituminous binder, maintaining the total thickness within 75 mm. These aggregates comply with the grading and physical criteria established to ensure durability and proper bonding. The binder types and application rates remain consistent with earlier stages, while construction follows detailed spreading, compaction, and finishing procedures.
Surface finish requirements include the use of specified binder types and grades selected per climate, and aggregates meeting physical and grading standards. Surface tolerances are tightly controlled, with limits on longitudinal and cross-profile variations. Quality control encompasses adherence to compaction, binder application, and aggregate distribution to ensure a durable and uniform base or binder course.
The built-up spray grout surface requires a wearing course prior to allowing traffic, as the open-graded construction is not immediately traffic-ready. Exceptions permit limited traffic with speed restricted to 16 km/h under supervision. Surface evenness and profile variations must meet specified tolerances before traffic is permitted.
Construction is prohibited when ambient shade temperature falls below 166C or during damp, foggy, rainy, or dusty conditions to ensure proper curing and bonding. The base must be dry and clean before binder application. Binder must be heated and applied at rates defined for each layer. Key aggregates are applied promptly after the second binder application. Surface finish tolerances and traffic opening restrictions are strictly enforced.
Frequently Asked
According to IRC 47 Clauses 4.1.1 and 4.1.2, recommended binders include straight-run bitumen complying with IS:73, road tar conforming to IS:215, and approved cutback bitumen. Suggested binder grades vary with climate and consist of 80/100, 60/70, and 30/40 for straight-run bitumen, and R.T.4 or R.T.5 for road tar. These binders are intended for use in base or binder courses but not as wearing surfaces.
Aggregates must be clean, durable, strong, and cubical, free from impurities such as salts, alkalis, vegetable matter, dust, and coatings. Physical properties include a maximum Los Angeles abrasion value of 50% for base course and 40% for binder course, aggregate impact values up to 40% and 30%, flakiness index capped at 25%, stripping values not exceeding 25%, water absorption limited to 1% (except slag), soundness loss under 12%, and minimum slag bulk density of 1120 kg/m3. Grading requirements specify percentage passing at various sieve sizes to ensure suitability for spray grout application.
Base preparation involves shaping and conditioning to a uniform grade and section, filling and compacting any depressions or potholes well ahead of construction. The surface must be dry and meticulously cleaned by sweeping, brushing with hard then soft brushes, and blowing to remove fine dust. Old blacktop surfaces require removal of sand, dirt, and loose debris. For stabilized soil or porous aggregates, a bituminous primer per IRC:16-1965 must be applied and cured before proceeding. Finally, the binder is heated and applied uniformly just prior to coarse aggregate spreading.
Construction begins with thorough base cleaning and preparation, applying a bituminous primer if needed. The first layer of coarse aggregate is spread at 0.5 m3 per 10 m8 and compacted with a suitable roller. Then, binder heated to specified temperature is applied at 12.5 to 15.0 kg per 10 m8 onto the dry aggregate surface. Immediately, the second aggregate layer is spread and compacted similarly. A second binder application at the same rate follows, after which key aggregates are placed at 0.13 m3 per 10 m8 and rolled to achieve a uniform surface. Proper weather conditions are essential, avoiding temperatures below 166C and damp or foggy environments.
Built-up spray grout is designed only for base or binder courses and is not appropriate as a wearing surface. As per IRC 47 Clause 5.11, a wearing course must be applied before the pavement is opened to traffic. In exceptional cases, limited traffic with a speed restriction of 16 km/h may be permitted immediately after construction under the Engineer-in-charge's discretion, but normal traffic should not be allowed until the wearing course is in place.
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